1. Technical Field
The present invention relates to dual-display liquid crystal display devices which enable displaying of two images concurrently so that the two images can be identified from two different viewing directions, respectively. More particularly, the present invention relates to a dual-display liquid crystal display device which includes sub-pixel areas each having therein a first storage capacitor section extending along a scanning line and a second storage capacitor section extending along a signal line, and further, makes the characteristics of brightness relative to a viewing angle associated with the two different viewing directions be line-symmetric with each other.
2. Field of Invention
Liquid crystal display displays, which are characterized by lightness in weight, small thickness and low power consumption as compared with cathode-ray tubes (CRTs), have been used for various types of electrical devices. A principle for displaying images employed in such a liquid crystal display device is such that, an amount of transmitted light or an amount of reflected light varies in accordance with alignment direction changes of individual liquid crystal molecules in proportion to the intensity of an electric field applied to the liquid crystal molecules, each of which is initially aligned in a prescribed direction resulting from a rubbing process performed on alignment films having the liquid crystal molecules interposed therebetween. Such an electric field is generated between each pair of electrodes to which a voltage difference is applied, and further, in order to allow viewers to identify images being displayed on the liquid crystal display device, it is necessary to configure a storage capacitor section corresponding to each pair of electrodes so as to retain the voltage difference for a prescribed constant time within a period of scanning time.
Meanwhile, in navigation devices, a dual-display liquid crystal display device is well known, in which, taking traffic safety into consideration, a first image, such as a navigation image, is configured to be displayed towards a driving seat, and concurrently therewith, a second image, such as a TV or DVD image, is configured to be displayed towards a front passenger seat. Among technologies which realize a liquid crystal display device in which, from different viewing directions, the corresponding different images can be identified, a technology employing a light-shielding film having slits formed therein (refer to JP-A-2006-184859), a technology employing a lenticular lens (refer to JP-A-2006-276591), and a technology employing a vertical alignment film which is formed on projections formed in a strip pattern (refer to JP-A-2006-276591), are publicly known. Hereinafter, a configuration of each storage capacitor section included in an existing liquid crystal display device will be described with reference to FIG. 10.
FIG. 10 is a plan view illustrating a storage capacitor section in one sub-pixel area of an existing liquid crystal display device.
In a liquid crystal display device 60 shown in FIG. 10, a lower storage capacitor electrode 61, which is one of electrodes forming a storage-capacitor section, is formed in each sub-pixel area. The lower storage capacitor electrode 61 is electrically connected to a storage capacitor line 62 which is formed so as to be parallel with a scanning line 67. A thin film transistor (TFT) operating as a switching element has a drain electrode D, which extends up to a position being overlapping, in plan view, the lower storage capacitor electrode 61 and forms an upper storage capacitor electrode 65. Further, the drain electrode D is electrically connected to a pixel electrode 63 via a contact hole 64. A storage capacitor section is formed of the upper storage capacitor electrode 65 and the lower storage capacitor electrode 61, and further, an insulating film (not shown in figures). In addition, in such a liquid crystal display device as disclosed in Japanese Patent No. 2,584,290, there is a case where the pixel electrode 63 composed of a transparent material is used as the upper storage capacitor electrode. Additionally, a portion denoted by a reference symbol 68 represents one of apertures formed in a barrier, which allow either of two images to be viewed from an oblique direction, in the case where the liquid crystal display device 60 operates as a dual-display liquid crystal device.
Forming a drain electrode and storage capacitor electrodes with a transparent conductive material leads to increasing of the resistance thereof, and, thus, makes it impossible to obtain a desired amount of capacitance, and further, results in arising of variation in the level of a voltage supplied to each pixel electrode and increasing of electric crosstalk. Therefore, in order to reduce the resistance, in general, the drain electrode and the storage capacitor electrodes are composed of an opaque metallic material, such as aluminum or an aluminum base alloy. However, forming the drain electrode and the storage capacitor electrodes with an opaque metallic material leads to a disadvantage in that an aperture ratio is decreased. Therefore, in order to increase the aperture ratio, in such a liquid crystal display device as disclosed in JP-A-9-120082, a method, in which a storage capacitor section is formed of two portions which extend along signal lines located at both sides of each sub-pixel area, respectively, and are disposed, in plan view, within a light-shielding area of a display panel, has been considered.
In general, in existing dual-display liquid crystal display devices, in order to ensure that a light ray emitted from each sub-pixel area in a viewing direction does not leak into light rays emitted from sub-pixel areas adjacent thereto in a different viewing direction, a light-shielding area is provided that is of dimensions larger than the dimensions of a light-shielding area for preventing light-ray leakage between adjacent sub-pixel areas, employed in general liquid crystal display devices. Therefore, in such a dual-display liquid crystal display device as disclosed in JP-A-2008-164740, a method, in which a storage capacitor line is formed so as to extend along a signal line and is located, in plan view, within a light-shielding area included in the dual-display liquid crystal display device, has been considered. This dual-display liquid crystal display device disclosed in JP-A-2008-164740 will be described below with reference to FIG. 11. In addition, in FIG. 11, elements the same as those included in the liquid crystal display device 60 shown in FIG. 10 are denoted by the same reference symbols, and further, detailed description of the same elements will be omitted below.
FIG. 11 is a plan view illustrating a one sub-pixel area of a dual-display liquid crystal display device disclosed in JP-A-2008-164740.
In a dual-display liquid crystal display device 70 shown in FIG. 11, a storage capacitor lower electrode 71 and a storage capacitor upper electrode 72, which form a storage capacitor section, are formed so as to extend along signal lines 66. The storage capacitor lower electrode 71 is formed by extending a drain electrode D composed of a metallic material, and similarly, the storage capacitor upper electrode 72 is formed by extending a storage capacitor line 73 composed of a metallic material. Therefore, both the storage capacitor lower electrode 71 and the storage capacitor upper electrode 72 are opaque, and thus, are formed within a barrier 69, in plan view, so as not to reduce an aperture ratio, and further, so as not to be exposed through an aperture 68 formed in the barrier 69 when viewed from an oblique direction.
However, in the dual-display liquid crystal display device 70 disclosed in JP-A-2008-164740, a configuration, in which the barrier (i.e., the light-shielding area) 69, which enables a liquid crystal display device to operate as a dual-display liquid crystal display device, covers only both side-edge portions but does not cover upper and lower portions of a one sub-pixel area leads to a problem due to the existence of a drain electrode D, which is specific to the dual-display liquid crystal display device 70. That is, in the existing dual-display liquid crystal display device 70, the drain electrode D of a TFT, which is electrically connected to a pixel electrode 63 via a contact hole 64 and is formed so as to have dimensions made as small as possible in order to increase an aperture ratio, does not make the shape of a display region of the sub-pixel area be symmetric with respect to a centerline between signal lines located at both sides of the sub-pixel area (refer to a portion “A” shown in FIG. 11), thus, does not make the characteristic of brightness in the dual-display liquid crystal display device 70, which is disclosed in JP-A-2008-164740, relative to a viewing angle be bilaterally symmetric, and, for example, causes a difference between the brightness of sub-pixel areas displaying a first image viewed from a viewing direction extending to a driving seat and the brightness of sub-pixel areas displaying a second image viewed from a viewing direction extending to a front passenger seat. Such a difference between the brightnesses of the two groups of sub-pixel areas viewed from two different directions causes two problems specific to the dual-display liquid crystal display device, one being a difference between the brightness of the two images viewed from the two viewing directions, the other one being a difference between a mixed color of three primary colors and a desired color, with respect to the two images viewed from the two viewing directions.